Control system for a sensor assembly

ABSTRACT

In a method of expelling contaminants from a sensor, a change is measured in a resonant frequency of a sensor assembly to detect contaminants on a surface of the sensor assembly. A change is measured in a frequency response of the sensor assembly to determine a presence and an amount of the contaminants on the surface. A cleaning mode is determined based on an amount of the contaminants on the surface. A cleaning is determined phase based on the amount of the contaminants on the surface. A cleaning control signal is provided to an actuator of the sensor assembly to expel the contaminants from the surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 15/605,463 filed May 25, 2017, which claims priority to U.S. Provisional Patent Application No. 62/454,154 filed Feb. 3, 2017, all of which are hereby fully incorporated herein by reference for all purposes.

BACKGROUND

This relates generally to a control system for a sensor assembly.

Obstacle and collision avoidance systems can be used to mitigate damage to vehicles and other property due to collisions. Various technologies regarding obstacle and collision avoidance systems can be incorporated into vehicles at a reasonable cost. Some technologies include sensors and digital cameras for sensing and monitoring areas around the vehicle. In some cases, cameras can increase safety by being mounted in locations that can give drivers access to alternative perspectives, which are otherwise diminished or unavailable to the driver's usual view through windows or mirrors.

SUMMARY

In a method of expelling contaminants from a sensor, a change is measured in a resonant frequency of a sensor assembly to detect contaminants on a surface of the sensor assembly. A change is measured in a frequency response of the sensor assembly to determine a presence and an amount of the contaminants on the surface. A cleaning mode is determined based on an amount of the contaminants on the surface. A cleaning is determined phase based on the amount of the contaminants on the surface. A cleaning control signal is provided to an actuator of the sensor assembly to expel the contaminants from the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example sensing and signaling control/monitoring system for a sensor assembly.

FIG. 2 illustrates an example for use on an exterior of a vehicle.

FIG. 3 illustrates an example plot illustrating a normalized natural frequency vs. normalized mass.

FIG. 4 is an example impedance magnitude response curve for an example actuator.

FIG. 5 illustrates example impedance magnitude response curves as a function of frequency for an example sensor assembly including different water droplet volumes disposed on the exposed surface of the example sensor assembly.

FIG. 6 illustrates an impedance magnitude response curve for an example non-faulty actuator and corresponding sensor assembly for different voltage excitation levels at around 300 kHz.

FIG. 7 illustrates an impedance magnitude response curve for an example faulty actuator and corresponding sensor assembly for different voltage excitation levels at around 300 kHz.

FIG. 8 illustrates an example impedance magnitude response curve for an example piezoelectric transducer and housing cover used in a sensor assembly for different temperatures.

FIG. 9 illustrates example impedance magnitude response curves for a frequency range of 20 to 40 kHz.

FIG. 10 illustrates an example plot that compares a temperature estimate using a linear equation for an example actuator and actual temperature from the impedance magnitude response plots of FIG. 9.

FIG. 11 is flow diagram illustrating an example method of expelling foreign contaminants from an exposed surface.

FIG. 12 is flow diagram illustrating an example cleaning process.

FIG. 13 is a flow diagram illustrating another example cleaning process.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

This description relates generally to a sensing and signaling control/monitoring system for sensors (sensor assembly) disposed externally on a vehicle. More specifically, this description relates to a sensing and signaling control/monitoring system for identifying contaminants, cleaning, temperature detection/regulation, fault detection, power regulation, etc. relating to sensors disposed externally on the vehicle. Ultrasound excitation for cleaning sensors provides a more cost effective and efficient approach than water sprayers, mechanical wipers, or air jet solutions. Thus, the sensing and signaling control/monitoring system utilizes an actuator that vibrates the sensor assembly and consequently, drives a contaminant (e.g., water, mist, ice, dirt, mud, etc.) deposited on an exposed surface of a sensor assembly at its resonant frequency so as to facilitate the removal of the contaminant from the exposed surface. More specifically, when the actuator is excited by the proper periodic waveform, the actuator will vibrate the sensor assembly. Properly adjusting the frequency and amplitude of the vibration will expel the contaminant from the exposed surface. Since different contaminant amounts and types result in different resonant frequencies, the actuator can provide a frequency in a range of frequencies that encompass the resonant frequencies of the combined sensor assembly and the amount of contaminant deposited on the exposed surface. Some example actuators include a piezoelectric transducer, a voice coil actuator, etc.

The sensing and signaling control/monitoring system can be utilized with any sensor device disposed externally on the vehicle. For example, some sensor devices include camera systems (e.g., camera monitoring systems (CMS), surround view systems (SVS)), photodetectors, external mirrors, reflectors, lasers (LiDAR). Other types of sensor devices may include short- and long-range radar, near-field transceiver, acoustic sensors or the like. Accordingly, the housing for the lens of cameras or other devices can include an exposed lens cover surface (e.g., camera, reflector, sensor, etc.). Similarly, other types of sensors (different from optical sensors or cameras) also include an external housing to protect the sensing devices from the environment. Each housing has an associated surface through which the signaling and/or sensing are provided to implement the corresponding sensing function (imaging, radar, LiDAR, near-field sensing, etc.). The sensing and signaling control/monitoring system not only cleans the housing, as described above, but also includes components to monitor other environmental or operating parameters for the sensor assembly. Examples of the environmental and operating parameters include temperature detection, fault detection (e.g., monitor the integrity and functionality of the exposed surface), power regulation, etc. The sensing and signaling control/monitoring system thus can extend the mechanical life of the sensor assembly and maintain its surface substantially free of contaminants. The sensing and signaling control/monitoring system may also provide early warnings for potential failures for the sensor assembly.

FIG. 1 illustrates an example sensing and signaling control/monitoring system 100, and FIG. 2 is an example sensor assembly 200 that can be used with the sensing and signaling control/monitoring system 100. The sensing and signaling control/monitoring system 100 includes a contaminant detection subsystem 110 that measures a resonant frequency of a sensor assembly, a fault detection subsystem 130 that detects faults in the sensor assembly based on the change in the frequency of the sensor assembly, a cleaning subsystem 140 that provides a cleaning control signal to an actuator in the sensor assembly to expel contaminants from exposed surfaces of the sensor assembly, and a temperature monitoring/power regulation device 160 that monitors a temperature of the actuator. A controller 170 is provided to control the subsystems and devices via a bus 190.

The example sensor assembly 200 illustrated in FIG. 2 is an example camera lens assembly for use on a camera. The sensor assembly 200 includes a housing 202 attached to a camera body 204, a sensing device (e.g., camera lens) 206 disposed in the housing 202, a transparent housing cover 208 disposed at an open end of the housing 202, and an actuator 210. The actuator 210 is disposed in the housing 202 and is attached to the housing cover 208. The actuator 210 includes electrodes 212 that allow the actuator 210 to be connected to the controller 170 via a circuit interface 214. In the example camera lens assembly, the actuator 210 can be a transducer (e.g., piezoelectric cylindrical or ring type transducer) that when excited by proper signaling, will vibrate the housing cover 208. As described herein, by correctly adjusting the frequency and/or the amplitude of the vibration the contaminants can be expelled from an exposed surface 216 of the housing cover 208.

Referring to FIG. 1 and FIGS. 3-5, the contaminant detection subsystem 110 detects and identifies contaminants disposed on an exposed surface of a sensor assembly (e.g., an exposed surface of a housing/lens cover) and can include a timer 112, a frequency measurement circuit 114, a frequency response measuring circuit 116, and a comparator 118. In one example, the contaminant detection subsystem 110 can be configured via the timer 112 to periodically check for contaminants based on various factors such as an amount of time the vehicle is in motion, a speed of the vehicle, direction of the vehicle (e.g., forward, reverse, turning), etc. The wait period can be dynamically updated (i.e. increased, decreased, no change) during the detection process. In another example, the contaminant detection subsystem 110 can be triggered manually (e.g., switch, push button, etc.) by an occupant of the vehicle. In yet another example, the contaminant detection subsystem 110 can be triggered by the sensor assembly if the sensor assembly senses that contaminants may be on the exposed surface. Thus, the triggering of the contaminant detection subsystem 110 can come from one of multiple sources.

Still referring to FIG. 1, the frequency measurement circuit 114 monitors a change in a resonant frequency of the sensor assembly to detect the presence of contaminants disposed on the exposed surface of the sensor assembly. A shift in the resonant frequency indicates that contaminants are present on the exposed surface of the sensor assembly. Specifically, the sensor assembly has a resonant frequency referred to as a natural frequency ω_(n) and is defined by Equation (1):

$\begin{matrix} {\omega_{n} = \sqrt{\frac{k}{m}}} & (1) \end{matrix}$

where k is the effective stiffness of a mechanical system (sensor assembly) expressed in N/m and m is the effective mass of the mechanical system expressed in kg. When contaminants are detected on the exposed surface, the resonant frequency changes from the resonant (natural) frequency of the sensor assembly to a resonant (natural) frequency of both the sensor assembly and the contaminants disposed on the exposed surface. A change in the natural frequency Δω_(n) due to the contaminants disposed on the exposed surface can be represented mathematically by Equation (2):

$\begin{matrix} {{\Delta \omega_{n_{-}{norm}}} = {{1 - {\sqrt{\frac{1}{1 + {\Delta m_{norm}}}}\mspace{14mu} {where}\mspace{14mu} {\Delta\omega}_{n\_ norm}}} = \frac{\Delta \omega_{n}}{\omega_{n}}}} & (2) \end{matrix}$

is a normalized change in natural frequency and

${\Delta m_{norm}} = \frac{\Delta m}{m}$

is a normalized change in mass both of which are unitless.

FIG. 3 is an example plot 300 that illustrates a change in the normalized natural frequency vs. a change in the normalized mass, as described above. The change in normalized natural frequency is very sensitive to a small change in normalized mass. In the example in FIG. 3, a change in normalized mass of about 10% results in a change in normalized natural frequency of about 70%. Thus, the sensitivity of the change in the resonant or natural frequency is effective in detecting the presence of contaminants on the exposed surface. Detection of the contaminants can be detected at a first resonant frequency, a second resonant frequency, etc.

Still referring to FIG. 1, the frequency response measurement circuit 116 measures a frequency response of the sensor assembly or any part thereof at a given resonant frequency to identify the type of contaminant and the amount of contaminant on the exposed surface. The frequency response of the sensor assembly plus the contaminants correlates to a specific amount of mass for contaminants on the exposed surface. For example, FIG. 4 is an example frequency response, which shows the impedance magnitude response curve 400 for an example sensor assembly. The peak is a pole of the impedance magnitude response and the valley is a zero of the impedance magnitude response. The pole-zero pair represent a resonant frequency of the example sensor assembly. More specifically, the pole represents a parallel resonant frequency and the zero a series resonant frequency. The term parallel resonant frequency refers to a resonance between the parallel combination of the mechanical subsystem and the dielectric whereas the term series resonance frequency refers purely to a resonance of the mechanical subsystem. FIG. 5 illustrates example impedance magnitude response curves 500 as a function of frequency for the example sensor assembly of FIG. 4 for different water droplet volumes ranging from 04, to 2004, disposed in the center of the exposed surface of the example sensor assembly.

As illustrated in FIG. 5, a given resonant frequency will shift by different amounts based on the amount of water (or other contaminants) disposed on the exposed surface. The amount of frequency shift from the resonant frequency of the sensor assembly correlates to an amount of mass on the exposed surface. Thus, in order to identify the type and amount of the contaminant on the exposed surface, the contaminant detection subsystem 110 can be calibrated with the resonant frequencies and frequency responses of the sensor assembly and any and all likely contaminant mass levels that may come in contact with the exposed surface. This information can be stored in a database 180 and the comparator 118 compares the measured resonant frequencies and frequency responses with the stored resonant frequencies and frequency responses to determine the amount and/or type of contaminants on the exposed surface of the sensor assembly. Identification of the contaminants can be initialized at a first resonant frequency, a second resonant frequency, etc.

Still referring to FIG. 1 and to FIGS. 6 and 7, the fault detection subsystem 130 performs system checks when the cleaning system 140 has not detected any appreciable mass on the exposed surface. System checks are performed by comparing a frequency response for a non-faulty, functional actuator and corresponding sensor assembly to a frequency response for a faulty, non-functional actuator and corresponding sensor assembly. For example, FIG. 6 shows the impedance magnitude response for an example non-faulty, functioning (healthy) actuator and corresponding sensor assembly for different voltage excitation levels at around 300 kHz. The response has a zero between 285 and 295 kHz and a pole between 305 and 310 kHz, depending on the voltage excitation level. FIG. 7, on the other hand, shows the impedance magnitude response for an example faulty or damaged actuator and corresponding sensor assembly for different voltage excitation levels at around 300 kHz. In this case, the zero near 290 kHz no longer has a resonant effect when the voltage level increases to the required level to excite the actuator. As a result, the impedance magnitude response of the sensor assembly can be monitored periodically during the life of the actuator. If the response indicates that the resonant frequency is no longer present, then the actuator is faulty and the fault detection subsystem 130 disables a system start signal (explained further below).

One example of a faulty actuator that the fault detection subsystem 130 can detect by a frequency response is the de-polarization of the piezoelectric material in a piezoelectric transducer when the transducer overheats. This failure occurs when the temperature of the material exceeds its Curie temperature and occurs when too much current is driven thru the sensor assembly during the cleaning process. Other example failures may include a cracked or broken lens, transducer cracking, seal failure, epoxy failure, etc. Thus, as explained in the previous paragraph, the frequency response for the faulty actuator can be compared to the frequency response when the actuator is not faulty. The frequency response(s) for non-faulty actuators can be stored in a database 180 and accessed to compare the faulty actuator frequency responses to the non-faulty actuator frequency responses.

Referring again to FIG. 1, the cleaning subsystem 140 initiates a cleaning process based on the identification of the contaminants by the contaminant detection subsystem 110. The cleaning subsystem 140 includes a cleaning mode selector 142, a cleaning phase selector 144, and a signal generation device 146. The cleaning mode selector 142 includes multiple cleaning modes (1, 2 . . . N) and selects a cleaning mode based on the type of contaminant disposed on the exposed surface as determined by the contaminant detection subsystem 110 described above. For example, a first cleaning mode may be implemented in response to determining the type of contaminants that correspond to mist, a second cleaning mode may be implemented in response to determining the type of contaminants that correspond to water droplets, a third cleaning mode may be implemented in response to determining the type of contaminants that correspond to ice, etc. Additional cleaning modes may correspond to other types of known (or unknown) types of contaminants such as dirt, mud, leaves, etc.

The cleaning phase selector 144 selects a cleaning phase from multiple cleaning phases (A, B . . . N) within a given cleaning mode based on the amount (e.g., size, mass, weight, volume, etc.) of contaminant disposed on the exposed surface as determined by the contaminant detection subsystem 110 described above. Thus, each cleaning mode can include one or more different cleaning phases depending on the amount of contaminants. Each cleaning phase within a given cleaning mode can provide a different level, intensity or process of cleaning based on the amount of contaminants on the exposed surface. Specifically, each cleaning phase can include one or more different parameters (i, ii . . . n) that define the cleaning process. The cleaning parameters can be defined as a frequency and/or voltage level that excites the actuator at specific resonant frequencies and/or amplitudes, which in turn vibrates the exposed surface thereby expelling contaminants from the exposed surface. Other parameters can include a time period (duration), heat drying, etc. Cleaning the exposed surface with ultrasonic systems and methods is described in copending U.S. patent application Ser. No. 15/492,286 filed Apr. 20, 2017, entitled METHODS AND APPARATUS USING MULTISTAGE ULTRASONIC LENS CLEANING FOR IMPROVED WATER REMOVAL, which is herein incorporated by reference in its entirety.

As described above, each cleaning phase can provide a different process of cleaning. For example, larger amounts of contaminants disposed on the exposed surface require a more aggressive cleaning than smaller amounts. For example, if the cleaning mode selector 142 selects a cleaning mode that corresponds to water, the phase selector 144 selects the cleaning phase that includes an appropriate number of cleaning parameters to efficiently expel the water from the exposed surface. More specifically, a first parameter can correspond to a first (high) frequency (e.g., about 300 kHz) that vibrates the actuator and hence, the exposed surface to atomize large water droplets. A second parameter can correspond to a second (lower) frequency (e.g., about 25 kHz) that vibrates the actuator to further expel smaller water droplets. A third parameter can correspond to using the transducer as a heating device to heat dry the remaining water droplets. Thus, during the cleaning process, as the amount of the contaminant on the exposed surface changes (decreases/increases), the cleaning phase and/or the cleaning parameter can change accordingly, (e.g., from a more aggressive cleaning process to a lesser aggressive cleaning process (or vice versa)) to efficiently remove the contaminant from the exposed surface. In other words, the voltage and/or frequency or any other parameter can vary during the cleaning process.

The signal generation device 146 generates a cleaning control signal 148 to an actuator via an actuator interface 150. The cleaning control signal 148 drives the actuator or other cleaning parameter based on the selected cleaning phase and/or cleaning parameters. The cleaning signal may have a predetermined frequency and/or voltage level that drives the actuator at the resonant frequency and/or amplitude to efficiently expel or dissipate the contaminants from the exposed surface. The cleaning signal can dynamically change as the cleaning mode, the cleaning phase, and/or the cleaning parameters dynamically change. As the contaminants begin to dissipate from the exposed surface, the resonant frequency of the exposed surface including the remaining contaminants changes. Thus during dissipation, the resonant frequency is essentially constantly changing. Therefore, as the resonant frequency changes, the cleaning mode, the cleaning phase and/or the cleaning parameters can change to continue efficient dissipation of the contaminants from the exposed surface that corresponds to the changing resonant frequency. In addition, the cleaning signal can be initiated at a first resonant frequency, a second resonant frequency, etc.

Referring again to FIG. 1 and to FIGS. 8-10, the temperature monitoring device 160 monitors a temperature of the actuator and can also serve as a power regulation device to regulate power to the actuator. Since the actuator is connected (e.g., mechanically coupled) to the exposed surface, the temperature monitoring device consequently monitors a temperature of the exposed surface. If the temperature of the actuator and/or exposed surface exceeds a threshold temperature the cleaning process is stopped until the actuator and/or exposed surface cools to ambient temperature or below the threshold temperature. Cooling can be passive cooling (e.g., air cool) or active cooling (e.g., air jets, water spray, etc.). In some examples, the temperature can be monitored by an external device, such as a thermocouple, infrared sensor, etc.

In another example, the temperature can be monitored internally by the sensor assembly. For example, the temperature monitoring device 160 can determine the temperature of the actuator and/or sensor assembly by measuring a frequency response of the sensor assembly for different temperatures. FIG. 8 illustrates an example impedance magnitude response 800 for an example sensor assembly at different temperatures. The magnitude of the impedance response at a particular frequency (e.g. 20 kHz) can be used to determine the temperature of the transducer. This information can be stored and can be accessed to determine the temperature of the actuator and if the temperature of the transducer exceeds a temperature safety threshold. If so, the cleaning process is stopped until the actuator and/or exposed surface cools to a safe operating temperature, which is below the threshold temperature.

FIG. 9 shows a close-up view of the impedance magnitude response 900 from 20 to 40 kHz. Given that the change in the impedance magnitude is uniform for a constant step changes in frequency, the temperature can easily be determined from the impedance data. In this example, the linear equation describing the temperature as a function of impedance magnitude for this example transducer is given by equation (3):

T=−0.29*Z+392.6  (3)

that has a coefficient of determination value of R²=0.9932. As this value approaches unity, the variance between the estimated value using the linear equation and the actual value is minimized.

FIG. 10 shows a plot 1000 that compares the temperate estimate using the linear equation and the actual temperature from the impedance magnitude plots. The maximum error in the estimated temperate is approximately 3.7° C. Thus, once an impedance magnitude value is known, the temperature of the actuator can be accurately estimated using a simple linear equation.

Referring again to FIG. 1, the controller 170 includes a microprocessor (microcontroller) 172 for executing instructions and/or algorithms to carry out the process of the sensing and signaling control/monitoring system 100. The microprocessor 172 can be embedded in a smart amplifier in a way such that the control system can be integrated into a single chip or can be comprised of multiple chips that are connected via bond wires. Logic control for the controller 170 can be software based (instructions executable by a processor core) or implemented as hardware, such as an arrangement of logic gates.

The controller 170 may further include a data storage device 174 that may store data and/or instructions such as executable program code that is executed by the microprocessor 172. The data storage device 174 may store a number of applications and data that the microprocessor 172 can execute to implement at least the functionality described herein. The data storage device 174 may comprise various types of memory modules, including volatile and nonvolatile memory. For example, the data storage device 174 can include one or more of random-access memory (RAM) 176, read-only memory (ROM) 178, flash solid state drive (SSD) (not shown), and a database 180. Additional devices and/or circuits 182, such as but not limited to pulse-width (PWM) switching controller(s), PWM pre-driver(s), amplifier(s), analog-to-digital convertor(s), multiplexor(s), etc. that facilitate execution of the signals regarding the actuator may be included.

FIG. 11 is flow diagram 1100 illustrating an example method of expelling the foreign contaminants from the exposed surface of the sensor assembly. The process begins in the contaminant subsystem described above. At 1102, the sensing and signaling control/monitoring system waits a period of time (e.g., waits for the system start signal) before beginning the process. At 1104, after the wait period has expired, the frequency measurement device monitors the resonant frequency to determine if contaminant(s) are present on the exposed surface. If no material is detected, then at 1106, the process proceeds to the fault detection subsystem where the sensing and signaling control/monitoring system undergoes a system check. At 1108, a decision is made to determine if the sensor assembly or any other component is faulty. If the system is faulty, then at 1110, the cleaning process stops. If the system is not faulty, the process loops back to 1102 and the process starts again.

If at 1104 material is detected, the contaminant detection subsystem 110 generates a material detection signal, then at 1112 the frequency response measurement circuit identifies the type of contaminant disposed on the exposed surface. At 1114, the process proceeds to the cleaning subsystem and the cleaning process is performed, which is further described below with reference to FIG. 12. At 1116, the temperature of the actuator is measured. At 1118, a decision is made to determine if the temperature of the actuator exceeds a temperature threshold. If “YES,” the process proceeds to the temperature monitoring subsystem where at 1120, the cleaning process is disabled. At 1122, cooling of the actuator and/or exposed surface is initialized. At 1124, a decision is made to determine if the actuator temperature still exceeds the temperature threshold. If “YES,” then at 1126 the cooling continues and the process loops back to 1124. If “NO,” then the process starts again at 1102.

If at 1118 the actuator temperature does not exceed the temperature threshold, then at 1128, a decision is made to determine if the cleaning process is complete. If “YES,” then the process starts again at 1102. If “NO,” then at 1130 the cleaning signal duration is updated and the process loops back to 1104.

FIG. 12 is flow diagram illustrating an example cleaning process represented as 1114 in FIG. 11. At 1202, the cleaning mode is determined based on the type of contaminant disposed on the exposed surface, as described above. At 1204, the cleaning phase is determined based on an amount of contaminants disposed on the exposed surface. At 1206, the cleaning parameters are set based on the phase selection. At 1208, the cleaning signal is generated to thereby initialize the cleaning process.

FIG. 13 is a flow diagram illustrating another example cleaning process represented as 1114 in FIG. 11. Prior to this process, it was determined that the contaminant identified on the exposed surface is ice and/or water. At 1302, the temperature of the actuator is determined, which in turn determines the temperature of the exposed surface. At 1304, a decision is made to determine if the temperature is below freezing, which is an indication that ice has formed on the exposed surface. If “YES,” then a heating signal is generated to heat the exposed surface to thereby melt the ice. If “NO,” then at 1308 a decision is made to determine if the amount of contaminant should be reduced. If “YES,” then at 1310 a cleaning signal is generated to excite the actuator at a resonant frequency of the exposed surface plus any contaminants on the exposed surface. If “NO,” then at 1312 a decision is made to determine if drying is required based on the amount of contaminant. If “YES,” then at 1314 a heat signal is generated thereby heating the exposed surface of the sensor assembly. If “NO,” then the process loops back to 1102 and starts the process over again.

Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims. 

What is claimed is:
 1. A method of expelling contaminants from a sensor, the method comprising: measuring a change in a resonant frequency of a sensor assembly to detect contaminants on a surface of the sensor assembly; measuring a change in a frequency response of the sensor assembly to determine a presence and an amount of the contaminants on the surface; determining a cleaning mode based on an amount of the contaminants on the surface; determining a cleaning phase based on the amount of the contaminants on the surface; and providing a cleaning control signal to an actuator of the sensor assembly to expel the contaminants from the surface.
 2. The method of claim 1, further comprising vibrating the actuator at a resonant frequency of the sensor assembly to expel the contaminants from the surface.
 3. The method of claim 1, further comprising determining whether a temperature of the actuator exceeds a temperature threshold based on the change in the frequency response of the sensor assembly, disabling the cleaning control signal and initiating a cooling procedure to cool the actuator responsive to the temperature of the actuator exceeding the temperature threshold.
 4. The method of claim 1, further comprising determining whether the actuator has a fault based on the change in the frequency response of the sensor assembly at different voltage levels, and disabling the cleaning control signal responsive to the fault. 